A simple hoist consists of a motor which raises and lowers a payload, typically under the control of an operator. The usual control for operating a hoist is a pushbutton pendant that allows an operator to control the hoist to raise and lower the payload up or down, sometimes with variable speed, and typically quite slowly (a few inches per second). For large payloads, such as a 300 pound engine block, an operator may be willing to live with the limits and restrictions on movement imposed by such a control, such as that the payload can only be moved up or down and only at a choice of two speeds.
For lighter payloads, however, agility of the hoist becomes of prime importance. If the load is a small glue gun or an automobile battery, an operator will not be so tolerant of inconvenient restrictions on movement. In repetitive motion environments, such as in a manufacturing assembly production line, even small loads can cause ergonomic problems due to long and repeated operator exposure to repetitive movements. Thus, it is not uncommon to use hoists or balancers for loads that are in fact easily within the range of human strength to lift unassisted in order to avoid fatigue or repetitive motion injuries of the operator. However, the operator's frustration with a hoist lacking the appropriate agility for the particular payload and job may cause the operator not to use the hoist and to eventually be injured. Even with larger loads, while operators will tolerate slow and clumsy control of payloads that they cannot lift unassisted, there are still great productivity gains to be had if a load can be more easily and "transparently" lifted by a really agile hoist.
The industry's response to the need for an agile, light-duty hoist is the balancer, which is a species of light hoist, although the terms hoist and balancer are not always clearly distinguished. The balancer provides a constant upward force on the payload equal to the payload's weight, thus "balancing" the payload against the force of gravity. The payload is effectively weightless and any additional forces applied by the operator to the payload will cause the payload to move up or down according to the applied force.
Generally, there are two popular kinds of balancers, spring balancers commonly used for small loads and pneumatic balancers often used for larger loads. Both types of balancers suffer from the problem that their upward force must be adjusted to match the weight of the expected payload. For balancing a tool, a "constant upward force" balancer is fine. But if the expected payload varies over the course of a task, for instance if the payload is picked up and later put down, the upward force that the balancer provides must be varied with the weight of the payload.
Typically, upward force is adjusted by adjusting the spring tension on the spring balancers, or by adjusting the compressed air pressure supplied to pneumatic balancers. For the right application, spring balancers are quite agile and responsive. They work well for counterbalancing a fixed payload such as a tool, however, they do not work so well for a varying payload because they cannot be easily adjusted "on the fly" to adjust for the varying load. Pneumatic balancers can be provided with two air pressure regulators with a pneumatic relay to switch between them, so that the balancer's upward force can be varied depending upon the task phase. Though pneumatic balancers can be "multiply tuned" for a load that changes during the course of a task, this adds significant complexity requiring multiple air pressure regulators and pneumatic relays, all of which require maintenance and adjustment.
Pneumatic balancers have a further problem in that they tend to have a broad "dead-band" in that a substantial amount of friction must be overcome to initiate the payload moving up or down. For instance, when the operator releases a load suspended by a balancer, it should not move the payload up or down. Of course, the upward force of the balancer and the force of gravity on the load typically won't always be perfectly matched which may result in drift of the payload up or down. Friction in the mechanism of the balancer may thus be helpful in preventing any drift or motion of the payload in this situation. In this sense, friction (or a simulacrum of it) is useful in preventing drift of the payload up or down. However, the greater the friction, the greater will be the "dead-band" or the amount of force the operator must apply to the payload to overcome the friction of the hoist and get the load moving.
In practice, spring balancers have little friction inherently. Pneumatic balancers tend to have too much friction and the resulting dead-band is broader than one would like. Note that the conventional hoists discussed earlier don't need any of the "tuning" that balancers need. Hoists move up or down as commanded by the operator control, regardless of the payload's presence or absence. However hoists don't have the agility of balancers, and they cannot be intuitively moved up or down by pushing on the payload itself. Instead, the operator must actuate switches to move the load up or down.
Electric motors have also been used in balancers to provide the necessary upward force to move the weight of the payload. By controlling the motor current, the motor's output torque can be controlled, which is converted to upward force by a reel which winds the cable from which the load is suspended. A control system for such a balancer can switch among different currents to control the motor to provide the appropriate amount of torque for different loads. Prior art hoist control methods enable the operator to manually switch between different potentiometer settings to control the current supplied to the motor, or provide a load cell to determine the weight of the load and automatically select between potentiometer settings to provide the proper counterbalance for the payload. Still needed, however, is a more effective way to control the adjustments to compensate for the weight of the payload. Efficient selection and use of motors is another issue with electric motor hoists and balancers. Generally it is desirable to use the smallest possible motor with enough power (power being the product of maximum speed and maximum torque) or more importantly, the smallest motor with enough torque for the largest expected payload. Unfortunately, electric motors tend to have more maximum speed than necessary and not enough torque to raise a payload at the relatively low speeds that a hoist typically moves a payload. To change torque and speed, a transmission is used which increases maximum torque by a factor of T and decreases speed by the same factor. The factor T is called the transmission ratio. Increasing T allows smaller, more cost effective motors to be used to move a payload.
Increasing T, however, also increases the friction of the hoist system as experienced from the load side. For a balancer friction can be a problem because the greater the friction, the more force must be applied by the operator to the load in order to overcome friction and cause the load to move. Depending on the quality of the transmission, beyond a certain value of T, the friction from the load side becomes essentially infinite: no matter how much force is applied to the output of the transmission back into the motor, the motor cannot be caused to turn. Friction in the system, as magnified by the transmission thus contributes to the width of the "dead-band" of the balancer or the amount of operator force that must be applied to the payload in order for it to move.
Needed is a lift assist device that addresses these issues with conventional hoists and balancers, and allows the use of more efficiently sized motors that can take advantage of larger transmission ratios. Further, it may have both a sensitive and responsive handle improving on the performance of hoists, and also a low dead-band "float-mode" improving on the performance of balancers.